The BBC's celebrated capacity to pull entertaining rabbits out of unprepossessing hats has led the British broadcaster on some peculiar enterprises over the decades, but I'll wager there have been few quite so curious as Torchwood, the Doctor Who spin-off series screened last year on Channel 10.

Torchwood is an organisation dedicated to confronting any alien threat to our gentle terrestrial idyll. According to the opening voiceover to each episode, Torchwood is "separate from the government, outside the police, beyond the United Nations: Torchwood sets its own rules". And it's based in Cardiff, Wales.

Its warrior caste consists of young adults with the brilliance of burning magnesium ribbon, the courage of lions and the resourcefulness of rogue traders. With such burdens of skill and responsibility, it's small wonder they protect themselves with a world-weary (and voluble) cynicism incongruous in ones so young. They are led by Captain Jack Harkness. The DVD case describes him as enigmatic, but he seems adrift somewhere between beaming evangelical innocent and leering anti-hero with a cute line in insincerity.

For me the characterisations don't work, but that's where the bad news ends. Torchwood's headquarters, a nice blend of modern office, 1950s nuclear laboratory and 1920s underground railway station hints at more interesting things to come, and the plots are nothing if not novel. In this, the final four episodes of the first series, the viewer discovers cannibals in the Brecon Beacons, an alien pendant that gives the wearer the ability to read the thoughts of others and an extra-terrestrial that sustains itself by drawing life from others. It's all quite gory but full of action. And you thought the verdant Welsh countryside harboured nothing more dangerous than the odd abandoned coal mine.

Torchwood is entertaining viewing, earthier and more sinister than its step-parent, Doctor Who. If only the cast didn't behave as if they were in their first year out of school uniform.